As printed circuit board and surface mount component geometries become smaller, it becomes even more important to maintain, throughout the manufacturing and especially the component attachment process, the precise alignment between the conductors on the surface mount device and the printed wiring board contact pads. Any undesired component displacements, especially skew, can cause product defects such as electrical open and short circuits. These defects necessitate increased spacing between components, increased product size, weight and cost.
Even under optimal process conditions, a surface mount component can be displaced from the desired location due to the forces acting on the component during the reflow soldering operation. For surface mount power devices incorporating a solderable heatsink, this skewing problem is more pronounced because of the large size of device and consequently large clearance areas require to accommodate even relatively small degrees of skew and because of their increased propensity to skew when large mounting pads are required to provide adequate heat transfer in the device.
One solution for the mounting of non-power surface mount devices on printed circuits is disclosed by Tribbey et al in U.S. Pat. No. 5,311,405. Tribbey teaches the use of a pair of tri-oval shaped areas in which solder paste is applied only to the elliptical areas within the tri-oval. When the surface mount device is placed thereupon, and the entire assembly is reflow soldered, the melting solder within the solder paste facilitates the alignment of the surface mount component with the pads. While this approach may be suitable for small devices, it is not well suited for power devices which have more than two leads and/or a very large heat dissipation pad.
In applications requiring high reliability for the mounting of power surface mount electronic components onto printed circuit boards, new solutions for these problems must be found. These problems are made even more difficult when the solution must be adaptable to high volume production techniques where product fallout and rework must be held to an absolute minimum.